In general, there is known an optical sensor device of the type including a light emitter for emitting light to a measurement target, and a light receiver for receiving the light having been reflected by or transmitted through a living body (see, e.g., Patent Documents 1 and 2). Patent Document 1 discloses a technique of illuminating a finger or an earlobe of a living body with light emitted from a light emitter, receiving the light having been reflected by or transmitted through the living body by a light receiver, and detecting a photo-plethysmographic signal corresponding to the pulse of the living body based on an electrical signal output from the light receiver. In the technique of Patent Document 1, an amplifier is connected to the light receiver in order to amplify the electrical signal obtained through photoelectric conversion performed by the light receiver, and the amplified electrical signal is input to a processor to execute various types of signal processing.
Patent Document 2 discloses a technique of reflecting light from a reference light source by a scanning mirror, and receiving the reflected light by a detection element. In the technique of Patent Document 2, a signal from the detection element is separated into a direct current (DC) component and an alternating current (AC) component, each of which is converted to a digital signal by an AD converter.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 6-22943
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2-13815
In the optical sensor device disclosed in Patent Document 1, the electrical signal output from the light receiver is amplified by the amplifier. At that time, because extraneous light, such as the sunlight, enters the light receiver in some cases, a noise component attributable to the extraneous light may be superimposed on the electrical signal. If the noise component attributable to the extraneous light becomes excessive, the amplifier would be saturated and a signal corresponding to the light emitted from the light emitter, such as a photo-plethysmographic signal, would not be detected correctively. Furthermore, if an amplification degree of the amplifier is reduced to prevent the saturation of the amplifier, a detected signal level would be reduced, thus causing a problem that sensitivity in light reception and detection accuracy of the photo-plethysmographic signal would degrade.
Moreover, conversion to the digital signal by the AD converter is required to execute the signal processing in the processor, etc. On that occasion, if the noise component attributable to the extraneous light is superimposed on the electrical signal from the light receiver, the electrical signal including the noise component is coded and, therefore, resolution of the AD converter has to be sufficiently increased with respect to a detection signal. For that reason, a dynamic range including the noise component as well needs to be prepared, thus causing another problem of raising the manufacturing cost.
On the other hand, Patent Document 2 discloses the technique of, after separating the signal from the detection element into the DC component and the AC component and amplifying them, converting each of those components to the digital signal by the AD converter. In the disclosed technique, however, the DC component and the AC component are separately subjected to signal processing, and an amplitude ratio between the DC component and the AC component is not restored to the same ratio as that when the signal has been output from the detection element. Accordingly, the converted digital signals cannot be directly applied to, for example, the case where the AC component is normalized using the DC component. In addition, in the optical sensor device disclosed in Patent Document 2, because the DC component and the AC component are detected in synchronism, the DC component and the AC component have to be converted to the respective digital signals by separate AD converters. Hence the manufacturing cost tends to increase.